Apparatus for translating the position of a writing instrument into electrical signals for transmission to a remote location such that the position, and corresponding movements, of the writing instrument may be recreated, are well known in the art. Thus, drawings, manuscripts, or the like, may be reproduced at remote locations. Among the more sophisticated prior art devices, are those in which movements of the writing instrument in the X and Y coordinates are sensed by electromagnetic means, or the like, and each sensed dimension is translated into a signal capable of transmission. X and Y coordinate positional information derived in the traditional manner may provide inputs to data processing apparatus such as computers, remote data terminals and special systems for processing coordinate data.
Some objections to some of the known art apparatus are limited resolving power, detrimental environmental effects, sensitivity to adjustment and instability and lack of accuracy to the degree which would be desirable. A number of other problems exist in these known systems including the need for a high density of grid wires for comparable performance and more complex circuits. As an example, most of these systems are both amplitude sensitive and phase sensitive, which places strict limitations on the inputs to the system. Another problem is that the spacing of the grid lines is extremely critical and very little variation is allowable. Accordingly, manufacture of the grid tablet is relatively expensive. A further problem relates to the critically of a coil diameter and the necessity of the position of the sensor being substantially parallel to the grid. Yet another problem with the known sensors is the fact that the cursor cannot be removed and replaced during a single operation, but must be initiated from the start if it is so removed.
Accordingly, it is an object of this invention to provide apparatus whereby the position of the cursor can be determined in a continuous linear fashion by using accurate electrical interpolation techniques to determine position between grids.
It is a further object of this invention to provide apparatus wherein the rate of counting is variable, thus providing any resolution desired.
Another object of this invention is to provide an apparatus wherein the accuracy of the output is not wholly dependent upon the scan rate.
Yet another object of the invention is to provide an apparatus which is substantially insensitive to amplitude and phase variation.
A still further object of this invention is to provide an apparatus wherein the diameter of the cursor coil is not critical.
A still further object of this invention is to provide an apparatus wherein some tilt of the cursor coil is permissible.
Another object of this invention is to provide an apparatus for obtaining absolute coordinate determination while permitting removal and replacement of the cursor from the grid tablet without re-initializing.
Still further objects of this invention are to:
provide a system without routine preventative maintenance requirement; PA1 provide a small number of wires per inch; PA1 provide a stable system without adjustments; PA1 provide a system relatively more immune to temperature, humidity, noise, dielectric variations, magnetization and electrical noise; PA1 provide a system with interchangeable subassemblies; PA1 provide a system with lower parts count and assembly labor. PA1 provide a system relatively immune to source (hard copy) material and thickness (except ferrous metals); and PA1 provide means to energize the grid wires by sharing the multiplexing wires at different positions in the tablet to minimize the feed wires required.